Devices are known with a surface on which is immobilized a layer of biomolecules having an affinity for other molecules ("the analyte") in a sample under test. Such devices are commonly referred to as biosensors. The immobilized biomolecules and the analyte may, for example, constitute a specific binding pair such as an antigen-antibody pair. Interaction of the two members of the pair causes a change in the physical properties of the device. This change can be used as an indicator of the presence and/or concentration of the analyte, the strength and/or progress of the interaction etc.
In many biosensors, it is the optical properties of the device which are monitored. One class of optical biosensor comprises a waveguide in the form of a thin layer of relatively high refractive index material coated on a substrate of optically transparent lower refractive index material. Biomolecules are immobilized on the surface of the waveguide and the interface between the substrate and the waveguide is irradiated with a beam of light.
Means are generally provided to facilitate coupling of light into the waveguide. The optical properties of the device will depend on the nature of those means, as well as on other factors including the wavelength of the incident light, the materials used for the waveguide and the substrate, the thickness of the waveguide etc. In general, however incident light is coupled to a greater or lesser extent into the waveguide. Chemical binding events at or in the vicinity of the waveguide surface will cause a localized change in refractive index, which in turn causes a change in the coupling characteristics of the device. This provides a means for monitoring interactions between the immobilized biomolecules and the analyte molecules.
One form of coupling means which has been proposed is a grating structure formed, for instance, in the interface between the substrate and the waveguide. In general, light incident will be reflected, transmitted or scattered into the various diffraction orders of the grating. Further, at certain angles of incidence, where a diffraction order matches the waveguide propagation condition, light will be coupled into the waveguide.
Here, light will propagate in the guide parallel to the substrate surface, where it will continue to interact with the grating. The light will couple back out of the waveguide via the various diffraction orders and into free-space beams. This outcoupled light will include beams in the same direction as the transmitted and reflected, uncoupled beams.
Attempts to measure the coupling condition are hampered by overlap of the waveguide derived beams and the uncoupled, transmitted or reflected components. This leads to measurements of low contrast.
One approach to this problem is to provide a pair of grating structures separated by an unmodulated region. Light incident on one of the gratings is coupled into the waveguide and is then coupled out by the second grating. The coupled-out light is thus spatially separated from the light reflected or transmitted at the first grating. However, the need for the provision of two gratings is a disadvantage.
In another approach, a single grating structure is employed, the grating structure being a superposition of grating elements having two different periodicities. Light incident on the grating structure at a first angle is coupled into the waveguide by the grating element with a first periodicity. It is then coupled out by the grating element having a second periodicity, at a different angle. The coupled-out light is thus angularly separated from the reflected light. Such a bidiffractive grating is relatively difficult to fabricate.
There haste now been devised methods for monitoring the interaction of molecular species, and devices suitable for use in such methods in which light is coupled into a waveguide by a grating structure, which overcome or substantially mitigate the above-mentioned disadvantages.